This invention relates to gasifiers as applied to biomass gasification for the production of a medium Btu grade fuel gas from a variety of biomass forms including shredded bark, wood chips, sawdust, sludges and other carbonaceous fuels or feedstocks.
The process system according to this invention relates to production of gas by use of a high throughput gasifier employing hot sand circulation for process heat. As is known in the art, the exothermic combustion reactions can be separated from the endothermic gasification reactions. The exothermic combustion reactions can take place in or near the combustor while the endothermic gasification reactions take place in the gasifier. This separation of endothermic and exothermic processes results in a high energy density product gas without the nitrogen dilution present in conventional air-blown gasification systems.
The present invention relates to a novel method of operating a gasifier preferably for a parallel entrained bed pyrolysis unit, i.e., a system comprising an endothermic reaction zone distinct from the exothermic reaction zone of the combustor wherein the heat from the exothermic zone is transferred to the endothermic reaction zone by circulation of an inert particulate solid such as sand. To be able to operate a gasifier in an entrained mode while using only low inlet gas velocities and at very high fuel feed rates would be an advance in the art and of commercial significance. The process disclosed by the present invention enables operating at inlet velocities typical of fluidized beds but operating in an entrained mode and with extraordinary thoughputs with fuel feed rates far above those contemplated as possible based on existing art.
The novelty and unexpectedness of the present invention can be readily ascertained by an examination of existing fluidization design equations and published literature information on the rates of biomass conversion to gas, particularly as regards the prediction of the design of a conceptual biomass gasifier operating at the conditions taught by the present invention.
Incorporated herein by specific reference is Chan, R. and Krieger, B. B., "Modeling of Physical and Chemical Processes During Pyrolysis of a Large Biomass Pellet with Experimental Verification", American Chemical Society, Division of Fuel Chemistry Preprints Volume 28, No. 5, August 1983, pp. 330-337 and in particular FIGS. 2 and 3 set forth therein.
The data on the rate of conversion of biomass to gas published by Chan and Krieger is used to estimate the residence time required to convert a substantial fraction of biomass to gas. The gas generation rate depends on the time the particle is exposed to approximately the same heat flux as used in a constant 1500 to 1600 F. fluid bed.
According to the published data of Chan and Krieger, to gasify woods chips should require, to dry, heat up, and pyrolize the wood, on the order of 2 to 3 minutes residence time in the gasifier. Heat balance calculations indicate that to provide the heat for gasification approximately 15 pounds of sand must be circulated per pound of wood gasified.
With this information, estimation can be made of the dimensions of a fluid-bed reactor to gasify wood. Sizing of a fluid bed using the prior art to predict the dimensions a fluid bed would have to have to operate at the high biomass throughputs taught by this invention will make all the more apparent the novelty and unobviousness of this invention.
Since wood and sand are constantly fed to the gasifier with sand and char withdrawn also at a constant rate, the residence time of wood and sand are equal. Because the sand does not react or change in weight, the sand flow and sand inventory in the reactor provide the design basis.
The residence time of wood and sand in the fluid bed is given by ##EQU1## The sand and inventory in the bed is given by EQU .rho..sub.B h.sub.B A.sub.B
where
.rho..sub.B =sand density at fluidization conditions, lbs/ft.sup.3 PA1 h.sub.B =fluid bed height, ft PA1 A.sub.B =cross sectional area of the fluid bed, ft.sup.2.
The sand feed rate is given by EQU W.sub.S (lbs/hr)=W.sub.So (lbs/ft.sup.2 -hr)A.sub.B (ft.sup.2).
W.sub.So is the specific sand throughput and determines the fluid-bed cross sectional area required to achieve the total sand feed rate which in turn is related to the wood rate by the heat balance. As mentioned, the heat balance requires approximately 15 pounds of sand per pound of wood. If one selects a wood throughput demonstrated to be feasible by this invention, for example, W.sub.Wo =2000 lbs/ft.sup.2 -hr where W.sub.Wo is the specific wood throughput, it is possible to estimate the fluid-bed height required to provide the necessary 2 to 3 minutes residence time. The expression for the residence time in terms of the above parameters is given by ##EQU2##
A reasonable value for the bulk density of a well fluidized bed of sand is common knowledge in the art and given by several fluidization texts at approximately 30 lbs/ft.sup.3.
Substituting in the above equation ##EQU3## indicates a bed height of approximately ##EQU4## would be required to provide the needed residence time at the high biomass throughputs taught by this invention. It is, however, well known by anyone familiar with fluidization technology that "slugging" occurs with long skinny fluid beds. The maximum fluid bed height to diameter ratio to avoid slugging is h/D=6 and preferably a ratio 2&lt;h/D&lt;6 is recommended for good fluidization.
Unlike the teachings of the prior art, the present invention is able to gasify 2000 lbs/ft.sup.2 -hr and even exceed 4500 lb/ft.sup.2 -hr through a unit of 10 inch (0.83 ft) diameter and length of 22 feet. Further, the operation is smooth and without any evidence of slugging. The present invention is therefore a radical departure from the teachings and conventional wisdom of the prior art. The prior fluidized bed art teaches away from this invention.